1. Field of the Invention
The present invention pertains to a system and method for compensating for effects of gas temperature fluctuations on respiratory gas sensing devices in airway flow measurement systems.
2. Description of the Related Art
It is well known to monitor the oxygen consumption or oxygen uptake of an individual for purposes of monitoring the physiologic condition of that person. The phrases “oxygen update” and “oxygen consumption” are used synonymously, and are both represented by the expression “VO2” or, for simplicity “VO2”. Oxygen consumption is a measure of the amount of oxygen that the body uses in a given period of time, such as one minute. It is typically expressed as milliliters of oxygen used per kilogram of body weight per minute (ml/kg/min). Measuring the rate of oxygen consumption is valuable, for example, in anesthesia and intensive care situations because it provides an indication of the sufficiency of a patient's cardiac and pulmonary function. VO2 can also be used to monitor the fitness of an individual or athlete.
VO2 is conventionally calculated as the difference between the volume of oxygen inspired and the volume of oxygen expired. The standard or direct calculation of VO2 is given by the following equation:VO2=Vi*FiO2−Ve*FēO2,  (1)where: “VO2” is oxygen consumption, “Vi” is inspired volume, “FiO2” is the inspired oxygen concentration, “Ve” is the expired volume, and “FēO2” is the mixed expired oxygen concentration.
An alternative method of calculating VO2 uses only the expired breath volume, Ve. In this scenario, the inspired breath volume Vi is calculated (rather than measured) based on the assumption that the nitrogen volume is the same for both inspired and expired gas, which is usually true because nitrogen is not consumed or produced by the body. This is referred to as the nitrogen balance. The calculation of Vi, rather than measuring it, also assumes that the effect of temperature and humidity are the same for both inspired and expired gas volumes.
This modification of equation (1), which uses a calculation of Vi based on the nitrogen balance noted above, is known as the Haldane transform. According to this technique, Vi is calculated as follows:Vi=Ve*FēN2/FiN2,  (2)where “FēN2” is the concentration of expired nitrogen, and “FiN2” is the concentration of inspired nitrogen. Based on this, the Haldane transform becomes:Vi=Ve*(1−FēCO2−FēO2)/(1−FiCO2−FiO2),  (3)and the oxygen consumption calculation becomes:VO2=Ve*[FiO2*((1−FēCO2−FeO2)/(1−FiCO2−FiO2))−FēO2],  (4)where FēCO2 is the expired carbon dioxide concentration, and FiCO2 is the inspired carbon dioxide concentration.
Calculating VO2 using the Haldane transform has the advantage that the effects of errors in volume measurements that are not “common mode” are eliminated, because only the expired volume measurement is used. Common mode errors are errors that effect both the Vi and Ve measurements, such as a calibration error in a flow sensor. Assuming, of course, the same sensor is used to measure Ve and Vi and the sensor performs consistently under variable conditions in its operating environment.
Mainstream sensors used to measure respiratory gas constituents can be subject to interference due to cyclical cooling effects of the flow of large volumes of gas past the sensor. Certain sensor technologies rely on one or more temperature controlled sensing elements placed within or near the gas stream. For example, certain oxygen sensitive elements located within the respiratory gas stream rely on the principle of fluorescence quenching to measure oxygen content in a flow, wherein the oxygen sensitive element is maintained at a constant temperature to obtain accurate measurements.
Because such an oxygen sensitive element must be in direct contact with the flowing respiratory gas, it is impractical to entirely eliminate or completely correct for temperature fluctuations resulting from respiratory flow changes. More generally, the operation of temperature controlled sensing elements can be affected by gas flows that have variable flow rates and that change direction, because temperature control systems may be unable to track flow-induced temperature changes with sufficient speed to avoid an adverse impact on the sensor operation.